Synthetic elastomers, e.g. ethylene-propylene terpolymers (EPDM), styrene-butadiene rubber (SBR), and butyl and nitrile rubbers, find widespread use, for instance in sealants, gaskets, conveyor belts and electrical cable insulation. In many of these applications they are preferred to natural rubber because of their superior resistance to degradation by heat, oils, solvents, oxygen and ozone.
Most synthetic elastomers belong to the non-crystallising category and, unlike natural rubber, their gum strength is poor. Useful mechanical properties can be achieved, however, by the addition of a particulate filler that improves the modulus and failure properties of the cured vulcanisate. This phenomenon is known as "reinforcement" and is associated with an increase in modulus, tensile strength and swelling resistance. Significant reinforcement is, in general, manifested only when the filler particles have a high surface area and the elastomer wets the filler.
Hitherto the most important and commonly used reinforcing filler has been carbon black. The so-called `structured` or reinforcing grades of carbon black are prepared from relatively expensive hydrocarbon feedstocks such as the aromatic fraction of petroleum or natural gas by burning in an oxygen-depleted atmosphere in special furnaces. Moreover, these feedstocks are in demand for the production of monomers (e.g. styrene) and other chemicals. Consequently, because of the competition for these limited hydrocarbon feedstocks, the cost of carbon black has tended to rise steeply. The search for cheaper alternatives to carbon black as a filler has thus been stimulated by the increasing costs of that material.
Synthetic calcium carbonates have attracted interest since their very small ultimate particle size suggests that they may be capable of providing a high degree of reinforcement in elastomer compositions. One advantage of these calcium carbonates is that they can be manufactured by processes that are not energy intensive and that involve raw materials of low cost. Thus, synthetic calcium carbonate of high surface area and of regulated particle size, shape and distribution and crystal structure can be prepared by the carbonation of an aqueous lime suspension. Another advantage is that the use of calcium carbonate as a filler permits the production of coloured or white elastomer composites.
However, in order to achieve adequate reinforcement, there should be a strong interaction between the surface of the filler particles and the elastomer leading to adhesion. Because of the hydrophilic nature of the surface of precipitated calcium carbonates (in contrast to carbon black), adhesion at the filler/elastomer interface is poor.
The surface treatment of precipitated calcium carbonates using stearic acid has been practised for many years. Although such a treatment increases the hydrophobicity of the calcium carbonate surface, the improvement in adhesion at the filler/elastomer interface is not significant and the performance characteristics of stearate-coated grades of precipitated calcium carbonate as rubber-reinforcing fillers are only modest.
In United Kingdom Patent Specification No. 1,603,300 (Imperial Chemical Industries), it is disclosed that the compatibility of a basic particulate filler with an organic polymer can be improved by coating the filler with an organic polymer which contains at least one unsaturated group and a carboxylic acid or carboxylic anhydride group. Such coated fillers are stated to be especially useful as fillers in natural or synthetic rubbers and they are referred to as "rubber-reactive fillers" in the art.
A rubber-reactive filler may be prepared in accordance with the disclosure of the aforesaid United Kingdom Specification by mixing a suspension of precipitated calcium carbonate with the triethylammonium salt of the methyl half-ester of a polybutadiene-maleic anhydride adduct. The reinforcing properties of a precipitated calcium carbonate coated with a carboxylated polybutadiene are discussed by J. Hutchinson and J. D. Birchall, Elastomerics, Volume 112, July 1980, page 17 et seq.
In the Journal of the Japanese Rubber Association (Nippon Gomu Kyokaishi)50, 7 (July 1977), pages 484-491, K. Shomato and co-workers describe the styrene cross-linking of an adduct polymer obtained by the reaction of maleic anhydride and liquid hydroxy-terminated polybutadiene, the cross-linking being effected in the presence of various inorganic fillers. Although the cross-linking reaction was noticeably hindered when carbon black was used as the filler, and the resultant cured elastomer had poor mechanical properties, the reaction proceeded smoothly with calcium carbonate as the filler and the mechanical properties of the resultant cured elastomer were better.